1. Field of the Invention
The invention relates to measuring gas operational turbine engine housing or casing temporary or permanent displacement or flexure and temperature with a distributed fiber optic sensing system (DFOSS) that utilizes optical frequency domain reflectometry (OFDR).
2. Description of the Prior Art
Gas turbine engines are assembled and subsequently validated for conformance with operational specifications, including the engine housing or casing displacement during operation. Engine housing displacement measurement accuracy during validation is on the order of < or =0.25 mm axially, and < or =0.1 mm radially. Engine housing displacements are currently measured using costly and time consuming image correlation photogrammetric methods. Many photographs have to be taken from different angles and compared to each other in order to calculate 3-D shape changes that occur in gas turbine casings. Reflective targets are typically placed on and around the casing and images of the casing are taken from many different positions. Each target is captured in multiple images and there are many targets in each image. The images are assembled using an image assembly computer program uses trigonometric formulas to tie all the images together and determine the locations of all the targets relative to each other. Often the photographs are taken manually and frequently over the whole course of a validation period (several months) by several photographers and technicians. In addition, because the turbine casing is almost always covered with thermal isolation mats that are removed before the photography sessions, the accuracy of such photogrammetric measurement campaigns with respect to actual operational temperature influenced casing displacement is limited. Photogrammetry can only measure displacement and does not provide information about casing strain or temperature during real world engine operation.
Known optical frequency domain reflectometry (OFDR) systems 8, such as shown in FIG. 1, based on Rayleigh scattering in optical fiber OF are capable of measuring with a reflectometer 9 the strain (c), temperature (T), and to some degree even the shape of an optical fiber OF or the component to which the fiber is attached. OFDR is a distributed measurement that results in measured information over the whole length of an optical fiber, which can be from several meters to several hundred meters long. Millimeter spatial resolution, high dynamic range, strain resolution of less than +/−1 microstrain, and temperature resolution of 0.1° C. can be achieved with today's technology. The Rayleigh scatter profile of a fiber OF is the result of reflection from random but static index variations that are inherent to any optical fiber. Thermal and mechanical effects on the fiber induce a frequency shift of the backscattered light that is proportional to the applied strain or temperature because fiber length varies with either application of strain or heat or both. Distance or spatial information can be encoded a frequency modulation-based measurement system. More particularly, as shown in FIG. 2, distance information is encoded in the frequency domain. In FMCW (Frequency Modulated Continuous Wave) ranging, the outgoing laser transmission is frequency modulated and the frequency difference between the reflected wave and the local oscillator wave is measured. From the measured frequency difference, also called the beat frequency, it is possible to determine the distance to the reflector. Commercially available OFDR/FMCW reflectometers and optical fibers (OF) can be obtained from Luna Innovations Incorporated of Blacksburg, Va., USA. However, if the known OFDR strain/temperature measurement systems and methods are applied to relatively hot gas turbine engine casing environments the known OFDR system output does not separate strain (e.g., turbine housing/case flexure influences) from the operational changes in temperature that cause change in optical fiber elongation.